Th.J Weijers

Bio: Th.J Weijers is an academic researcher from Philips. The author has contributed to research in topics: Orthogonal polynomials & Harmonic. The author has an hindex of 2, co-authored 2 publications receiving 54 citations.

Finestructure of triode characteristics

Abstract: The detailed structure of diode-, triode- and tetrode-characteristics is experimentally investigated by measuring the amplitudes of the different harmonics (up to the tenth) in the anodecurrent, as a function of the grid bias potential, when a sinusoidal e.m.f. of constant amplitude is applied to the grid. These curves show a great number of maxima and minima. The amplitude of every next harmonic, considered as a function of the grid biassing potential has, to a pronounced degree of accuracy, the form of the differential of the former one. It is thus possible to determine the successive differentials of the characteristic in a wide domain. However, in some small domains this appears not to be the case. The latter anomalies in the higher differentials of the characteristic are most likely due to critical potentials in the secondary electron emission from the electrodes. A new analytical development of the characteristic in a series of Tchebycheff polynomials is given which has some advantages over the development in a Taylorseries. Whereas the coefficients of the Tchebycheff series follow immediately from the measurements with a finite amplitude on the grid, the coefficients of the Taylorseries can only be derived from the limit, towards which these measurements converge for an infinitely small gridswing.

Above-surface neutralization of highly charged ions: The classical over-the-barrier model

Abstract: The neutralization of highly charged ions during interaction with metallic surfaces is accompanied by the ejection of a large number of secondary electrons Recent experiments demonstrate two main contributions to this electron ejection process: one from the region below the surface and the second from the above-surface portion of the trajectory We present a theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model The theory incorporates resonant multielectron capture of conduction electrons, resonant loss into unoccupied states of the conduction band, and intra-atomic Auger deexcitation The effective barrier potential includes quantum corrections to the classical image potential The effect of below-barrier (``tunneling'') transfer is investigated The solution of a coupled system of rate equations allows the approximate determination of the n-shell populations, the projectile charge state, and the total number of Auger electrons The calculation describes the transient formation of ``hollow'' atoms We find satisfactory agreement with recent data for K Auger yields by Meyer et al [Phys Rev Lett 67, 723 (1991)]

Measurement and interpretation of swarm parameters and their application in plasma modelling

Abstract: In this review paper, we discuss the current status of the physics of charged particle swarms, mainly electrons, having plasma modelling in mind. The measurements of the swarm coefficients and the availability of the data are briefly discussed. We try to give a summary of the past ten years and cite the main reviews and databases, which store the majority of the earlier work. The need for reinitiating the swarm experiments and where and how those would be useful is pointed out. We also add some guidance on how to find information on ions and fast neutrals. Most space is devoted to interpretation of transport data, analysis of kinetic phenomena, and accuracy of calculation and proper use of transport data in plasma models. We have tried to show which aspects of kinetic theory developed for swarm physics and which segments of data would be important for further improvement of plasma models. Finally, several examples are given where actual models are mostly based on the physics of swarms and those include Townsend discharges, afterglows, breakdown and some atmospheric phenomena. Finally we stress that, while complex, some of the results from the kinetic theory of swarms and the related phenomenology must be used either to test the plasma models or even to bring in new physics or higher accuracy and reliability to the models. (Some figures in this article are in colour only in the electronic version)

Determination of the townsend. Ionization coefficient α for mixtures of neon and argon

Abstract: fr Resume En outre on a determine les quantites γ (nombre d'electrons supplementaires sortant de la cathode a la suite du choc d'un ion positif) et V (une correction a ajouter a la tension d'amorcage, calculee avec les valeurs de, α et γ). Etant donne que plusieurs publications anterieures signalent une augmentation plus ou moins en forme d'escalier du courant photoelectrique pour un champ constant en fonction de la distance des electrodes on a etudie particulierement cet aspect des experiences.

Real-Time Time-Dependent Electronic Structure Theory.

Abstract: Real-time electronic structure methods provide an unprecedented view of electron dynamics and ultrafast spectroscopy on the atto- and femtosecond time scale with vast potential to yield new insights into the electronic behavior of molecules and materials. In this Review, we discuss the fundamental theory underlying various real-time electronic structure methods as well as advantages and disadvantages of each. We give an overview of the numerical techniques that are widely used for real-time propagation of the quantum electron dynamics with an emphasis on Gaussian basis set methods. We also showcase many of the chemical applications and scientific advances made by using real-time electronic structure calculations and provide an outlook of possible new directions.

Monte Carlo studies of non-conservative electron transport in the steady-state Townsend experiment

Abstract: An investigation of the spatial relaxation of the electrons and benchmark calculations of spatially resolved non-conservative electron transport in model gases has been carried out using a Monte Carlo simulation technique. The Monte Carlo code has been specifically developed to study the spatial relaxation of electrons in an idealized steady-state Townsend (SST) experiment in the presence of non-conservative collisions. Calculations have been performed for electron transport properties with the aim of providing the benchmark required to verify the codes used in plasma modelling. Both the spatially uniform values and the relaxation profiles of the electron transport properties may serve as an accurate test for such codes. The explicit effects of ionization and attachment on the spatial relaxation profiles are considered using physical arguments. We identify the relations for the conversion of hydrodynamic transport properties to those found in the SST experiment. Our Monte Carlo simulation code and sampling techniques appropriate to these experiments have provided us with a way to test these conversion formulae and their convergence.